1. Field of the Invention
The present invention relates to a method of inspecting an active matrix substrate including driving circuits for a liquid crystal display apparatus or the like.
2. Description of the Prior Art
In recent years, an active matrix substrate for a liquid crystal display apparatus has become greatly miniaturized and fabricated with high accuracy. In some cases, thin-film-transistors (TFTs) in a pixel portion, a scanning line driving circuit and a data line driving circuit are formed on one substrate. The provision of driving circuits in an active matrix substrate has advantages over a case where the driving circuits are externally mounted. The advantages are that lower production cost can be realized, that it is possible to correspond to fine pitches, and that the panel module size can be made smaller. By making use of these advantages, a liquid crystal display apparatus in which an active matrix substrate includes driving circuits having a size of about 1 inch (2.54 cm) has been commercially available for a view finder.
Generally, driving circuits included in an active matrix substrate are required to operate at a high speed. For example, in the National Television System Committee (NTSC) system, a scanning line driving circuit is required to operate at 15.75 kHz, and a data line driving circuit at a few megahertz. Therefore, polysilicon having higher mobility than amorphous silicon is used for TFTs which form part of a driving circuit. FIG. 21 is a block diagram showing a conventional liquid crystal display apparatus using polysilicon TFTs. The liquid crystal display apparatus comprises a pixel portion 11, a data line driving circuit 12, a scanning line driving circuit 13, a common line 11b for the connection to a counter electrode (not shown), video lines 12a, analog switches 12b, and a shift register 12c.
An exemplary method of fabricating such an active matrix substrate is described below. A thin polysilicon film with a thickness of about 1000 .ANG. (100 nm) is first formed on a quartz glass substrate by low pressure chemical vapor deposition (CVD). Next, Si ions are injected into the thin polysilicon film, so that the thin polysilicon film is made to be amorphous. The thin amorphous polysilicon film is heat-treated in a nitrogen atmosphere at a temperature of 600.degree. C., for example, for about 100 hours, so as to obtain a thin polysilicon film. The thin polysilicon film is patterned to form TFT channel layers and lower electrodes for additional storage capacitors. The resistance of the lower electrodes is reduced by injecting P ions. Thereafter, an SiO.sub.2 gate insulating film with a thickness of about 1000 .ANG. (100 nm) is formed by low pressure CVD at a temperature of about 850.degree. C. On the entire surface of the SiO.sub.2 gate insulating film, a polysilicon film with a thickness of about 5000 .ANG. (500 nm) is formed by low pressure CVD. The resistance of the polysilicon film is reduced by diffusion, after N.sup.+ polysilicon is deposited at a temperature of about 850.degree. C. Then, the polysilicon film is patterned to form gate electrodes, scanning lines, and upper electrodes for the additional storage capacitors and wiring thereof. After forming N.sup.+ source and drain regions by injecting P ions, an SiO.sub.2 layer-insulating film is deposited to have a thickness of 7000 .ANG. (700 nm) by atmospheric pressure CVD. Then, contact holes are formed, and Al is deposited by sputtering. Patterning is performed to form data lines. Next, after an SiO insulating film is deposited entirely on the top surface of the substrate on which patterns have been formed by plasma CVD, through-holes are formed for the contact of pixel electrodes with the drain. Indium-tin-oxide (ITO) is deposited to have a thickness of 1000 .ANG. (100 nm) by sputtering, and then patterning is performed to form pixel electrodes. In the above processes, CMOS peripheral circuits such as a scanning line driving circuit and a data line driving circuit are formed at the same time.
A liquid crystal display apparatus is assembled by using the thus fabricated active matrix substrate and a counter substrate which are positioned opposite to each other with a liquid crystal layer interposed therebetween. After the liquid crystal display apparatus is assembled and can actually be driven, the active matrix substrate can easily be inspected optically. However, if the active matrix substrate in the assembly is proved to have a fault, it is very difficult to correct the fault in a small-sized panel. Furthermore, the assembling process of the active matrix substrate with the counter substrate is in vain. Therefore, it is required to inspect the active matrix substrate after TFTs, etc. are formed.
To meet this requirement, in a conventional method of inspecting an active matrix substrate, needle-like probes are directly brought into contact with signal lines and the conductive conditions between signal lines are checked. Alternatively, there has been a non-contact inspection method or an inspection method in which an inspecting circuit is provided on the substrate. Examples of such conventional inspection methods are disclosed in Japanese Laid-Open Patent Publication Nos. 57-38498 and 60-2989.
However, by the inspection method in which needle-like probes are directly brought into contact, a problem has occurred that, since each pixel portion of an active matrix substrate fabricated in small size and with high accuracy is minute in size, there is an increase in probability that pixels are damaged or that another breakage of line will occur as a result of the inspection. By the non-contact inspection method, a problem has occurred that, since portions to be inspected are large in number and minute in size, it is not always easy to reliably inspect all the portions at a high speed. By the inspection method in which an inspection circuit is provided on the substrate, a problem has occurred that, since the inspection circuit itself may have a fault due to a faulty transistor or the like for connection, the inspection cannot be reliably performed.